This invention relates to the production of nickel powder, and more particularly to a new and improved method for producing nickel powder of controlled geometry, such as powder surface area, particle size, and particle shape, for use in conductive or resistive pastes or inks or other uses and processes where surface area or particle size of the metal powder is important.
While nickel powders have been made by many processes, none has, to my knowledge, been controlled as to particle geometry. Probably the best known method of producing nickel powder has been the pyro-reduction of nickel carbonyl, [Ni(CO).sub.4 ], where the relatively elevated temperature results in particle growth and particle-particle sintering.
Experience with precious metals in the electronics industry indicates that chemical precipitation methods give the possibility of controlling the metal powder geometry. In considering some of the precipitating methods that have been used sucessfully for noble metals such as platinum or palladium, it is noted that the chemical activity of the reducing agent required to reduce a base metal compound to metal powder is much more than is required to reduce a noble metal compound. This is because base metal compounds are much more strongly bound together than noble metal compounds, and also base metal compounds usually precipitate as other compounds rather than as metals. The noble metals are unique in their ease of precipitation in metallic or uncombined condition.
Hydrazine is recognized chemically as a very powerful reducing agent and its use with noble metal compounds has been well documented. It also has been used for nickel compound reduction to nickel metal powder. However, the procedures taught by the prior art do not provide nickel powder of controlled geometry.
For example, Sulzberger, in U.S. Pat. No. 1,164,141 precipitates nickel, or cobalt, powder from several salts of nickel by the use of hydrazine or salts thereof and employs a platinum group metal salt to "incite" the reaction. This results in a combination metal powder, i.e. nickel plus palladium or platinum, which may, in many instances, be unsuitable for the envisioned uses of the nickel powder prepared according to the method of the present invention. In addition, the Sulzberger process does not provide nickel powder of controlled geometry.
Sharov et al. in articles abstracted by Chemical Abstracts (Volumes 64 and 65, 1966) disclose a process for producing nickel powder involving the precipitation from nickel hydroxide [Ni(OH).sub.2 ] by means of hydrazine. This, however, is specific for the hydroxide as specified by the equation given in the second article: EQU Ni.sup.2+ + N.sub.2 H.sub.4 + OH.sup.-.fwdarw. Ni + N.sub.2 + H.sub.2 O
the reduction product is also specified in the second article as (Beta)Ni + Ni(OH).sub.2 and in the first article, the percentage is specified as 93-6% metallic nickel. This product, containing 4-7% Ni(OH).sub.2 may be unsuitable for the envisioned uses of the nickel powder prepared according to the method of the present invention. In addition, the Sharov et al process does not provide nickel powder of controlled geometry.
Gershov et al, in an article abstracted by Chemical Abstracts (Volume 78, 1973), discloses an autocatalytic method of reducing nickel or cobalt chloride by the use of hydrazine which requires temperatures of 100.degree.-140.degree. C which, in turn, require a pressure vessel to accomplish the reduction. No control of particle geometry is offered.
Accordingly, while a number of procedures of the prior art have been used to precipitate nickel metal from solutions or slurries of nickel salts by means of hydrazine and/or its salts, none of these procedures provides a nickel powder wherein the geometry such as powder surface area, particle size and particle shape is controlled.